The present invention generally relates to communication equipment and more particularly, to a radio frequency signal combining/sorting apparatus which composes (combines) a plurality of transmission signals for transmission to a common output line or antenna.
A recent trend, in mobile unit communication systems such as automobile telephones, etc. utilizing a frequency band region of 800 MHz has been to employ, a so-called, cellular system, in which a number of radio frequency channels corresponding to the radio traffic capacity of the cells (wireless zones) are provided in one base station.
By way of example, in a mobile unit communication system for an automobile telephone or the like, which recently has experienced a rapid increase in the number of users, a large number of channels, as many as 32 to 64 channels for example, are required in one base station.
In a case where so many radio frequency channels are to be provided in one base station, the use of an antenna sharing technique at the base station is essential from an economical point of view, and development of an efficient radio frequency signal combining/sorting apparatus for combining many input signals into one output signal has been strongly demanded.
The radio frequency signal combining/sorting apparatus disclosed herein is not limited in its application to such an antenna sharing device as referred to above, but may also be employed generally in a power composing (combining) device having a construction as shown in FIG. 15 or 16.
In FIG. 15, a known power combining device PA includes so-called 3dB hybrid circuits H1,H2 and H3 connected to each other and respectively grounded through resistors R1,R2 and R3 which serve as dummy loads, input terminals IN1 and IN2 for the 3dB hybrid circuit H1, input terminals IN3 and IN4 for the 3dB hybrid circuit H2, and an output terminal OUT led out from the 3dB hybrid circuit H3.
In the above arrangement PA, the 3dB hybrid circuit H1 combines the power of the signals inputted to the input terminals IN1 and IN2 for application to one input of the 3dB hybrid circuit H3, while the 3dB hybrid circuit H2 combines the power of the signals inputted to the input terminals IN3 and IN4 to be applied to the other input of said 3dB hybrid circuit H3. Thus, the 3dB hybrid circuit H3 subjects the both signals thus inputted to power combination and outputs the same.
Another known power composing device PB in FIG. 16 includes input terminals IN1,IN2,--and INn respectively coupled to channel filters F1,F2,--and Fn each constituted by a band-pass filter, through isolators I1,I2,--and In, a power composing circuit (or junction unit) JU coupled with said channel filters F1,F2,--and Fn, and an output terminal OUT led out from the circuit JU.
In the power composition circuit PB as described above, respective signals inputted to the input terminals IN1,IN2,--and INn are prevented from mixing with other inputs by the isolators I1,I2,--and In, and the signals passing through channel filters F1,F2,--and Fn are subjected to power composition by the junction unit JU for output-therefrom through the output terminal OUT.
The so-called 3dB hybrid composing system shown in FIG. 15 is a simplified system, without frequency characteristics in principle. However, since half of the power is absorbed by dummy loads each time the power passes through the 3dB hybrid circuit, it is not generally used as the signal combining/sorting apparatus for effecting power transmission.
On the other hand, the so-called junction unit composing system shown in FIG. 16, which employs the channel filters for passing the respective band regions of the predetermined channel frequencies by inputting signals of the corresponding channels, the power composition may be effected with a small sharing loss. Therefore, this system is generally employed as the radio frequency signal combining/sorting apparatus.
The relationship between the respective channels and transmission characteristics of the respective channels are shown in a graphical diagram of FIG. 17, in which center frequencies of the respective channels and channel filters are represented by f1,f2,f3,--and fn. As is seen from FIG. 17, in order to reduce interference with respect to neighboring channels, a Q value above a predetermined constant value is also necessary, while high frequency temperature stability is required for suppressing any increase of the insertion loss due to displacement of the central frequency by temperature changes. However, in a case where the arrangement is applied to a system as having a channel interval of 100 KHz, for example, in a band region of 800 MHz to 1.5 GHz, it is difficult to construct a channel filter having a stable frequency characteristic, with a high Q value, maintained even in a cavity resonator, semi-coaxial cavity resonator or dielectric resonator, etc., and thus, an increase in the insertion loss can not be avoided.